From: Andrea Corallo <akrl@sdf.org>
To: Alan Mackenzie <acm@muc.de>
Cc: Rocky Bernstein <rocky@gnu.org>,
Stefan Monnier <monnier@iro.umontreal.ca>,
emacs-devel <emacs-devel@gnu.org>
Subject: Re: Correct line/column numbers in byte compiler messages
Date: Sat, 21 Mar 2020 16:28:55 +0000 [thread overview]
Message-ID: <xjf7dzdlliw.fsf@sdf.org> (raw)
In-Reply-To: <20200321153041.GA7805@ACM> (Alan Mackenzie's message of "Sat, 21 Mar 2020 15:30:41 +0000")
Alan Mackenzie <acm@muc.de> writes:
> Hello, Andrea.
>
> On Sat, Mar 21, 2020 at 11:22:03 +0000, Andrea Corallo wrote:
>> The outcome as I see it is that total bootstrap time gets bigger 1.1x
>> while normal runtime appears not affected.
>
> Well, it looks like the normal runtime is around 2.x% slower for
> scratch/accurate-warning-pos.
Well I studied physics so for me 2% is pretty much zero :) :) Joking
apart I'm not sure this is really sufficient to conclude is noise or
not.
>> For my quick understanding of how it works this is expected. The
>> additional branch and compare against symbols_with_pos_enabled in `eq'
>> is a kind of branch that is very easily predictable by any modern CPU,
>> therefore when the feature is off (not compiling) it becomes transparent
>> (I'd see a compiler branch hit there too).
>
> In other words, the processor will test symbols_with_pos_enabled
> simultaneously with starting the continuation for the "not" case.
The processor will just speculate guessing the target branch without
having to wait for symbols_with_pos_enabled value to be loaded. Given
this change rarely, speculation there should be pretty much always
correct.
I'd wrap symbols_with_pos_enabled into something like:
#define SYMBOLS_WITH_POS_ENABLED \
__builtin_expect(symbols_with_pos_enabled, 0)
To make sure we minimize instruction cache overhead too.
> This extra test in the EQ code was always the main thing in the slowdown
> occurring in this git branch.
Is the EQ overhead the main/only one? Also GC seems marginally affected.
I think would be interesting to write a nano benchmark EQ focused to
test this accurately.
> When I timed things back in 2018, I got a slowdown of somewhat more than
> 2.x%. May I ask what sort of processor you're using? Mine (unchanged
> since then) is an AMD Ryzen.
I did the test on a "Xeon E5-1660 v3". I think we can classify it as a
good system from few (6?) years ago. Not very fast by today's standards
but still quite beefy in terms of caches.
Bests
Andrea
--
akrl@sdf.org
next prev parent reply other threads:[~2020-03-21 16:28 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-19 15:10 GNU is looking for Google Summer of Code Projects Rocky Bernstein
2020-03-19 17:35 ` Stefan Monnier
2020-03-19 17:56 ` Andrea Corallo
2020-03-19 18:05 ` Andrea Corallo
2020-03-19 18:19 ` Rocky Bernstein
2020-03-19 21:26 ` Stefan Monnier
2020-03-19 21:45 ` Andrea Corallo
2020-03-19 23:07 ` Rocky Bernstein
2020-03-19 20:34 ` Correct line/column numbers in byte compiler messages [Was: GNU is looking for Google Summer of Code Projects] Alan Mackenzie
2020-03-19 20:43 ` Andrea Corallo
2020-03-20 19:18 ` Alan Mackenzie
2020-03-21 11:22 ` Andrea Corallo
2020-03-21 15:30 ` Correct line/column numbers in byte compiler messages Alan Mackenzie
2020-03-21 16:28 ` Andrea Corallo [this message]
2020-03-21 18:37 ` Andrea Corallo
2020-03-21 20:19 ` Alan Mackenzie
2020-03-21 21:08 ` Andrea Corallo
2020-03-21 23:39 ` Andrea Corallo
2020-03-22 11:26 ` Alan Mackenzie
2020-03-19 20:56 ` Correct line/column numbers in byte compiler messages [Was: GNU is looking for Google Summer of Code Projects] Rocky Bernstein
2020-03-19 22:05 ` Stefan Monnier
2020-03-20 19:25 ` Alan Mackenzie
2020-03-19 21:41 ` Stefan Monnier
2020-03-19 22:09 ` Stefan Monnier
2020-03-20 20:10 ` Alan Mackenzie
2020-03-20 21:23 ` Rocky Bernstein
2020-03-20 21:27 ` Clément Pit-Claudel
2020-03-20 23:46 ` Stefan Monnier
2020-03-20 21:30 ` Stefan Monnier
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